A tunneling magneto-resistive (TMR) element has a structure in which a tunnel barrier layer is sandwiched between two ferromagnetic layers. When an external magnetic field is applied and the relative magnetization angle between the two ferromagnetic layers is changed, the tunneling conduction probability of an electron through the tunnel barrier layer is changed and the resistance of the TMR element is changed. Such a TMR element is applied to a read-out sensor part of a magnetic head used for a hard disk and a device such as a nonvolatile memory MRAM utilizing magnetism.
An oxide of aluminum (Al), titanium (Ti), magnesium (Mg), or the like is used as a material for the tunnel barrier layer of the TMR element. Particularly a magnesium oxide (MgO) tunnel barrier layer can have a larger magneto-resistance change rate (MR ratio) (refer to non-patent document 1).
Manufacturing methods of the MgO tunnel barrier layer include a method of forming the MgO layer directly by radio-frequency (RF) sputtering of an MgO target and a method of depositing an Mg layer and thereafter forming the MgO layer by oxidation treatment.
For a TMR element using the method of forming the MgO tunnel barrier layer directly by the RF sputtering of the MgO target, a method is disclosed (refer to non-patent document 2), which performs substrate heating after the formation of the MgO tunnel barrier layer, as a technique to improve the MR ratio particularly for a low RA (element resistance×element area).
Further, for a TMR element using the method of depositing the Mg layer and forming the MgO layer by the oxidation treatment, a method is disclosed (refer to patent document 1), which after depositing a first Mg layer, forms an MgO layer on the surface of the Mg layer by natural oxidation, and then deposits a second Mg layer, thereby forming a tunnel barrier layer including the first Mg layer/MgO layer/second Mg layer.
As another method, there is disclosed a method which after depositing a first Mg layer, performs oxidation treatment under a high pressure, thereafter deposits a second Mg layer and performs oxidation treatment under a low pressure (refer to patent document 2).
Moreover, it is disclosed to form a stacked body of a first MgO layer and a second MgO layer as the tunnel barrier layer (refer to patent document 3). In the method disclosed in patent document 3, first, a first Mg layer is formed and a first MgO layer is formed by oxidation of the first Mg layer. Then, the first MgO layer is caused to have a crystalline orientation by annealing of the first MgO layer in a magnetic field. Successively, a second Mg layer is formed on the first MgO layer and the second Mg layer is oxidized to form a second MgO layer, and thus a tunnel barrier layer is formed as the stacked body of the first MgO layer and the second MgO layer.